Frame Relay Network Performance

Transcription

1 Lab 4 Frame Relay Network Performance Overview Frame Relay is a connection-oriented, unreliable technology based on virtual circuits. A virtual circuit must be set up between a source and destination before any data may be exchanged. Most Frame Relay service providers support Permanent Virtual Circuits (PVC), which are long-lived connections, as opposed to Switched Virtual Circuits (SVC), which are set up at call time. When a PVC is set up, certain parameters which govern the behavior of the PVC are agreed upon. These include the Committed Information Rate (CIR), committed burst rate (Bc), and the excess burst rate (Be). The CIR is the rate (in bits per second) which the service provider guarantees to the customer. The committed burst rate specifies how quickly that data may be sent. For instance, a customer may contract for a CIR of 64 Kbps, and a Bc of 32 Kbits/100 ms. In this case, the customer may send 64 Kbits per second, but no more than 32 Kbits in any 100 ms period. Frame Relay service providers will accept additional data above the CIR and transmit it if there is excess capacity in the network at that time. The amount of excess data that they will carry is defined by the excess burst rate. Any data frames that are sent in excess of the CIR are marked with the Discard Eligible bit set. These frames are discarded first in the event of congestion. Objective To examine the behavior of a Frame Relay PVC as contract parameters are varied. Build the Simulation Model Start up OPNET IT Guru Academic Edition. First, it would be useful to read the information provided by OPNET regarding Frame Relay configuration. Select the File tab => Open. Choose the Frame_Relay project and click on OK. Read through the Objective, Network Configuration, and Glossary pages. Select File => Close when you are done. Select the File tab => New... Choose Project and click on OK. Change the Project Name to xx_wan_frame_relay (where xx are your initials). Set the Scenario Name to CIR64 and click on OK. In the Initial Topology window, select Create Empty Scenario and click on Next. In the Choose Network Scale window, select World and click on Next. In the Choose Map window, choose usa and click on Next. In the Select Technologies window, scroll down and include the frame_relay_advanced model family, and then click on Next. In the Review window, click on OK.

2 OPNET Lab Manual First, we will configure a bursty data application to generate traffic over our frame relay network. Transfer Protocol (FTP) will serve our purposes. The File Select an Application Config object from the Object Palette and place it in the project workspace. Right click on the object and choose Edit Attributes. Set the name attribute to Bursty Application. Expand the Application Definitions attribute and set the rows attribute to 1. Expand the row 0 attribute and set the Name attribute to FTP_Bursty. Expand the Description attribute and edit the value of the Ftp attribute. Set the Inter-Request Time (secs) attribute to exponential(0.1). Set the File Size (bytes) attribute to constant(1000). In both cases, you will need to set the Special Value field to Not Used in order to modify the attribute values. The application you have now defined will transfer an average of ten 1-KB files every second. The exponential distribution is often used to model waiting times so we can use it here to model the time between file requests. Click on OK to close the window. 38

3 Lab 4 Frame Relay Network Performance Select a Profile Config object from the Object Palette and place it in the project workspace. Right click on the object and choose Edit Attributes. Set the name attribute to Bursty Profile. Expand the Profile Configuration attribute and set the rows attribute to 1. Expand the row 0 attribute and set the Profile Name to FTP_Bursty_Profile. Expand the Applications attribute and set the rows attribute to 1. Expand the row 0 attribute and set the Name to FTP_Bursty. Set the Repeatability attribute to Once at Start Time. Click on OK to close the window. 39

4 OPNET Lab Manual Select an fr32_cloud object from the Object Palette and place it in the project workspace. Right click on the cloud and choose View Node Description. The cloud represents a WAN that consists of Frame-Relay-capable switches and that supports up to 32 Frame Relay connections. Right click on the cloud and select Set Name. Set the name to Frame_Relay_Cloud. Click on OK to close the window. Select an fr_wkstn_adv device from the Object Palette and place it in the project workspace. Right click on the station and choose View Node Description. Note that the station supports the Frame Relay protocol. We will now set up the client to use the bursty FTP application that we defined. Right click on the station and choose Edit Attributes. Modify the name attribute of the device to Bursty Client. Edit the Application: Supported Profiles attribute. Insert a row and set the Profile Name to FTP_Bursty_Profile. Click on OK to close the window. 40

5 Lab 4 Frame Relay Network Performance Select an fr_server_adv device from the Object Palette and place it in the project workspace. Right click on the server and choose View Node Description. Note that the server also supports the Frame Relay protocol. We will now set up the server to support the Bursty FTP application that we defined. Right click on the device and choose Edit Attributes. Modify the name attribute of the server to Bursty Server. Edit the Application: Supported Services attribute. Insert a row and set the Name to FTP_Bursty. Click on OK to close the table window. Click on OK to close the window. 41

6 OPNET Lab Manual Select two fr8_switch_adv objects from the Object Palette. Place one next to the client and one next to the server. Right click on one of the switches and choose View Node Description. Note that the switch takes action when it becomes congested. It will discard the frames marked with the Discard Eligible Bit first, but may even discard normal frames. Click on the close window icon to close the window. Right click on the client s switch and select Set Name. Set the name to Client Switch. Click on OK to close the window. Right click on the server s switch and select Set Name. Set the name to Server Switch. Click on OK to close the window. Select three FR_T1_int links from the Object Palette, and use them to connect the client to its switch, the server to its switch and the two switches to the Frame Relay cloud. Remember that T1 speed is 1.5 Mbps. 42

7 Lab 4 Frame Relay Network Performance Finally, we can set up a permanent virtual circuit (PVC) between the client and the server so that they can communicate. Select an FR PVC Config object from the Object Palette, and place it in the project workspace. Right click on it and select Set Name. Set the Name to PVC Config. The FR PVC Config object must be placed in the workspace before any PVCs may be set up. Click on OK to close the window. Select an fr_pvc object from the Object Palette and use it to connect the client to the server. When you right click to finish placing the PVC, choose Abort Demand Definition. Right click on the PVC, and expand the Contract Parameters attribute. These parameters constitute the agreement between the end systems (client and server) and the Frame Relay service provider. Set the Outgoing CIR (bits/sec) attribute to Set the Outgoing Bc (bits) to Set the Outgoing Be (bits) to The Committed Information Rate (CIR) is the amount of traffic that the service provider has agreed to carry. The committed burst size (Bc) indicates how much data can be sent in a given interval. In this case, the interval is one second, so the CIR = Bc. The excess burst size (Be) is the amount of traffic that the service provider may accept in addition to the committed rate, if network conditions allow. Excess data may be discarded if the service provider s network becomes congested. Click on OK to close the window. 43

8 OPNET Lab Manual Configure and Run the Simulation Select the Simulation tab => Choose Individual Statistics Expand the Global Statistics item and the Frame Relay item, and select the Delay (sec), Delay Variance, and Residual Error Rate statistics. Expand the FTP item, and select the Download Response Time (sec) and Traffic Received (bytes/sec) statistics. 44

9 Lab 4 Frame Relay Network Performance Expand the Node Statistics item and the Frame Relay PVC item, and select the BECN Status, DE Status, and FECN Status statistics. Click on OK to close the window. 45

10 OPNET Lab Manual Select Simulation => Configure Discrete Event Simulation Under the Common tab, set the Duration to 5 and the unit to minute(s). Click on OK to close the window. 46

11 Lab 4 Frame Relay Network Performance Duplicate the Scenario We are now going to duplicate the scenario and configure the PVC to support a higher data rate. This will allow us to compare the performance across the two PVC rates. Choose Scenarios => Duplicate Scenario, and name the new scenario CIR128. Right click on the PVC, and expand the Contract Parameters attribute. Set the Outgoing CIR (bits/sec) attribute to Set the Outgoing Bc (bits) to Leave the Outgoing Be (bits) unchanged at Click on OK to close the window. Run the Simulation Select the Scenarios tab => Manage Scenarios Edit the Results field in both rows and set the value to <collect> or <recollect>. Click on OK to run both scenarios (one after the other). When the simulation has completed, click on Close to close window. Inspect and Analyze Results Select the Results tab => Compare Results Select and expand the Global Statistics item and the Ftp item, and select the Download Response Time (sec) statistic. Use As Is mode to view this statistic. Click on Show to see a more detailed graph. The statistic shows how long each download took, and there will be one point for each file downloaded. You can see that the downloads took much less time in the CIR128 case and that the variation in download times was much less. This is to be expected since there was more capacity available to the application in the second scenario. Click on the close window icon and choose to Delete the panel. Click on the statistic again to disable the preview. 47

12 OPNET Lab Manual Expand the Traffic Received (bytes/sec) statistic and view using As Is mode. Click on Show to see a more detailed graph. This statistic shows the total amount of FTP traffic received by both the server and client. You can see that more traffic is received in the CIR128 case, and that there is more variation than in the CIR64 case. Click on the close window icon and choose to Delete the panel. Click on the statistic again to disable the preview. 48

13 Lab 4 Frame Relay Network Performance Select and expand the Global Statistics item and the Frame Relay item, and select the Delay (sec) statistic. Use time_average mode to view this statistic as this mode will make it easier to see the difference in performance. Click on the Show button to see more detail. The statistic shows how long each frame took to be delivered. You can see that the delay is greater in the CIR64 case and that the delay is growing over time. This is due to longer delays within the switches due to full buffers. Click on the close window icon and Delete the panel. Click on the statistic again to disable the preview. Expand the Delay Variance (sec) statistic and view using As Is mode. Click on Show. This statistic shows the variance in frame delivery delay. The CIR64 case shows much more variability and the variability is growing over time. Click on the close window icon and Delete the panel. Click on the statistic again to disable the preview. 49

14 OPNET Lab Manual Expand the Residual Error Rate statistic and view using As Is mode. Click on Show. This statistic shows the percentage of frames which are discarded due to congestion or corruption. You can see that many more frames are discarded in the CIR64 case and that the percentage is growing as the switch becomes more and more congested. Click on the close window icon and Delete the panel. Click on the statistic again to disable the preview. Select and expand the Object Statistics item, the Bursty Client item, and the Frame Relay PVC item. Select the DE Status statistic and view using As Is mode. Click on Show. This statistic shows the number of frames which were received by the client which had the DE bit set. When many frames are received with the DE bit set, that indicates that the source of the frames was frequently sending faster than the contracted CIR. You can see that far more frames are marked with the DE bit in the CIR64 case. This makes sense since the application produces the same amount of traffic in both scenarios, but the CIR128 scenario provides more capacity for carrying the traffic. Click on the close window icon and Delete the panel. Click on the statistic again to disable the preview. 50

15 Lab 4 Frame Relay Network Performance The BECN Status or FECN Status statistics show how many frames were received with the BECN or FECN bits set. These bits are set by the switch when it is congested. If you view these statistics, you will see that no frames were received with the bits set. The offered traffic is not sufficient yet to cause congestion at the switches. Save your model and close all windows. Questions 1. How much traffic is being generated by the Bursty Server? You can calculate this data rate using the Inter- Request Time and File Size parameters you set when configuring the bursty application. 2. Given the traffic level calculated in Question 1, explain how this traffic can be carried given the speed of the links used to connect the devices, and the contract parameters of the PVC connecting the client and server. 3. Duplicate the CIR64 scenario, and replace the T1 link between the client and its switch with an FR_DS0_int link. Remember that DS0 speed is 64 Kbps. Rerun the simulation and inspect the BECN Status and FECN Status statistics for both the Bursty Client and Bursty Server. How do they compare to the CIR64 scenario results? Explain your results. Explain the usage of the BECN and FECN bits in the Frame Relay header. Also inspect the Residual Error Rate statistic. How does it compare to the CIR64 case? 4. Frame Relay service providers often offer Frame Relay PVC contracts called zero CIR. In these cases, all packets are marked discard eligible. Zero CIR service is usually less expensive, but provides fewer guarantees. Duplicate the CIR64 scenario, and edit the PVC attributes. Set all the Contract Parameters to zero. Rerun the simulation and compare your results to the CIR64 results. What differences did you find? What would be the effect on the Traffic Received at the Bursty Client if you added another client with a CIR of 64 Kbps to the zero CIR case? Explain your answer. 5. In Question 3, some frames are discarded due to congestion at the switch. The congestion can be relieved if the traffic source transmits frames less frequently. Frame Relay systems provide traffic shaping algorithms to restrain the traffic source from sending too quickly. Duplicate the DS0 scenario from Question 3, and edit the 51

16 OPNET Lab Manual Bursty Server attributes. Expand the Frame Relay Traffic Shaping attribute, and set the Status to Enabled. Click on the question mark to the left of each of the traffic shaping attributes to determine their function. Calculate values for these attributes that will result in an uncongested network; in other words, restrict the data rate to less than the speed of the DS0 line (64 Kbps). Write down the function of each attribute, the value you calculated, and an explanation of the calculations. Run the simulation and gather statistics to show that you achieved the goal of reducing congestion (and corresponding frame discards). 52